Change ServerRequest.attribute(String) to return Object

This commit changes `ServerRequest.attribute(String)`` to return
`Optional<Object>` instead of `Optional<T>`, where `T` was infered
from a type parameter.

spring-test/src/main/java/org/springframework/mock/web/reactive/function/server/MockServerRequest.java

@@ -156,12 +156,6 @@ public class MockServerRequest implements ServerRequest {
 		return (Flux<S>) this.body;
 	}
 
-	@SuppressWarnings("unchecked")
-	@Override
-	public <S> Optional<S> attribute(String name) {
-		return Optional.ofNullable((S) this.attributes.get(name));
-	}
-
 	@Override
 	public Map<String, Object> attributes() {
 		return this.attributes;